Genetic Linkage

Key Concepts: Linked Genes and Genetic Mapping

Linked Genes

• Definition: Genes that are located on the same chromosome and tend to be inherited together.
  
  

• Why Are They Inherited Together?
  Alleles for linked genes are less likely to be separated during meiosis because they are physically close to each other on the same chromosome.
  
  

  • Crossing over (the exchange of genetic material between homologous chromosomes during meiosis I) can sometimes separate linked genes, but the closer the genes are, the less likely they will be separated by crossing over.

• Examples of Linked Genes:
  Some traits like hair color and eye color may be inherited together, as the genes for these traits are often linked. For instance, blonde hair and blue eyes, or brown hair and brown eyes, tend to appear together more frequently due to the linkage of these genes.
  
  

Sex-Linked Genes

• Definition: Genes located on the sex chromosomes (X or Y).

• Most sex-linked genes are found on the X chromosome, as the Y chromosome has fewer genes.

  • X-linked genes are the most common reference when discussing sex-linked traits.

Constructing a Linkage Map

• Purpose: A linkage map shows the relative positions of genes on a chromosome based on how frequently they are inherited together.

• How to Construct a Linkage Map:

  • Observe the Frequency of Crossing Over: Measure how often two genes are inherited together versus how often they are separated due to crossing over.

  • Data Required: To construct a linkage map, you would need:

    • The genetic distances between genes based on crossover frequency.
      
      

    • The percentage of gametes that have both genes from the same chromosome (i.e., showing recombinant frequencies).
      
      

  • Key Point: Genes that are very close together on the chromosome will have a low frequency of crossing over (less than 50%), while genes located far apart will have a higher chance of crossing over (about 50%).

Linkage Map Example:

A linkage map for the X chromosome could show the locations of genes, such as those associated with hemophilia A and G6PD deficiency, which are located close together and show a lower frequency of crossing over. On the other hand, genes that are farther apart, like Protan and Xm, would show a higher frequency of crossing over.

Summary

• Linked Genes: Genes that are close together on the same chromosome, inherited together more often.

• Sex-Linked Genes: Genes on the X or Y chromosome, most commonly discussed as X-linked.

• Linkage Map: A genetic map constructed using the frequency of crossing over between genes, showing the relative locations of genes on a chromosome.

Review Questions

1. What are linked genes?
   Genes located on the same chromosome, which are inherited together unless separated by crossing over.
   
   

2. Explain how you would construct a linkage map for a human chromosome. What data would you need?
   To construct a linkage map:
   
   

   • You need crossing over frequency data (how often genes are inherited together vs. separately).

   • The lower the crossing over frequency between two genes, the closer they are on the chromosome.

3. People with red hair usually have very light skin. What might be a genetic explanation for this observation?
   The genes for red hair and light skin are likely linked genes on the same chromosome, so they are often inherited together.
   
   

4. How often does crossing over occur between non-linked genes? Explain your answer.
   For non-linked genes, crossing over occurs about 50% of the time during meiosis because genes on different chromosomes assort independently.